Because of the significant expense involved with drilling oil and gas wells, it is desirable to determine such characteristics as the pressure, permeability, and invasion diameter of a subsurface formation in order to determine the ability of the well to produce before committing further resources. For example, formation pressure data is important for evaluating the extent of the reserves and the permeability of the formation is important because it is needed to develop an economical production plan. Much work has been done over the years in developing techniques and down hole tools to make these determinations. In one conventional method for determining the characteristics of subsurface formations, the well is cased down to the producing formation, or even through the formation, and perforated to allow fluid entry. Ordinarily, the well stands full of drilling fluid, or water, to control the escape of valuable fluids from the producing formation. A string of tubing is lowered into this well, the tubing having a valve at its base. This valve is ultimately located essentially at the top of the producing formation. A second valve is located at the top of the drill string which leads to a surface pressure measuring device, often a deadweight tester. There can also be a bottom hole pressure measuring device, called a pressure bomb, which can be either internal plotting, or surface recording.
Testing was generally divided into three parts for cased formations. The first part involved measurement of the initial formation pressure by using a pressure bomb to determine bottom hole pressure before formation fluid was drawn. This was followed by a three day flow test to allow formation fluid to flow to the surface for rate determination at a constant rate. The final portion of the test was a six-day pressure build-up test in which the well was shut-in and the bottom hole pressure recorded versus time, so that the formation flow capacity and skin effect could be determined.
It was found that it was necessary to shut the wells in at the bottom of the tubing string for low to moderate permeability gas wells. This was generally done using some type of controllable tubing valve, and preferably employing a packer on the outside of the tubing to close the annulus at the top of the production formation. This second procedure was preferred instead of shutting in the well at the top. Shutting-in the well at the top takes much longer in low permeability formations to reduce the flow of fluid into the well to a low enough value to allow for analysis of the build-up pressure curve. While such a method was somewhat satisfactory, it suffered from the disadvantages that: (1) the measurement of fluid flow rates were notoriously poor for low permeability formations; and (2) the total testing time was too long, for example, on the order of about 6 to 10 days, or more.
In situations where the borehole is open (not cased), especially when the formation is relatively soft, the above procedure is not practiced because of time restraints. That is, in open wells, because the testing time often exceeds an hour, there is fear that the walls of the borehole will cave-in and trap the drill string. Thus, there would be a great advantage if the measurements needed to determine the characteristics of a formation could be performed in only a matter of minutes. The present invention provides such an advantage.
An improvement to the above technique for cased-in wells is disclosed in U.S. Pat. No. 4,423,625, which teaches a so-called "limited volume well bore transient test". Formation fluid flows into a volume of known dimensions in a down hole test tool and the rate of pressure increase is measured with time. Such a method supposedly permits calculation of flow rates from knowledge of the properties of the fluid, the temperature of the gas, and the volume into which it is flowing. Although the method disclosed in this '625 patent did substantially decrease the test time, it still took from about 12 to 24 hours to complete the test, which is much too long for successfully testing a formation in an open well.
Consequently, there still exists a great need in the art for a method and apparatus which will increase the accuracy and reduce the time for making formation pressure measurements, especially in low permeability formations from open wells.